1. Field of the Invention
The invention relates to an optical fiber coupler for optical amplifier use in the field of optical communications, a process for fabricating the optical fiber coupler and an optical fiber amplifier using the same.
2. Prior Art
In the field of optical communication systems, optical fiber amplifiers have been developed to directly amplify laser beam signals.
An optical fiber amplifier utilize an principal that a rare earth doped fiber amplifies a signal laser during the population inversion of rare earth ions dispersed in the fibers which is pumped up by pumping light which is introduced the fiber. In such an optical fiber amplifier, a optical fiber coupler has been used to introduce the signal light and the pumping light. U.S. Pat. No. 5,802,224 by M. Okuta et al. discloses an optical coupler in which two optical fibers are connected partially on the sides of the fibers to form a fusion-welded and extended portion. The above U.S. Patent discloses use of two optical couplers which is connected by a rare earth-doped fiber to apply to an optical fiber amplifier in which said portion is used to mix signal light passing though one of the two fibers with pumping light passing through the other fiber from the pumping light laser source. The mixed light in the fusion-welded and extended portion is extracted from either of fibers at the opposite end of the portion to the said signal entering fibers and transmitted to the rare earth doped fiber to amplify the mixed light.
Such a optical coupler has been known, for example, in U.S. Pat. No. 5,802,224 by T. Ozeki which discloses a light distributor comprising a plurality of optical fibers having a tapered portion which is formed by thermally fused together to form a light mixing portion where the cores of the fibers are packed close to each other in a single cladding.
A conventional structure of the optical fiber amplifier is shown in FIG. 7, including a multiplexer 7 and a rare earth-doped optical fiber 4 doped with rare earth metal such as Er or Nd in the fiber core, which is connected to the multiplexer, a pumping light source 6 connected to the multiplexer 7, and a fiber for entrance of a laser signal to the multiplexer 7. The rare earth-doped optical fiber 4 functions as a laser beam amplifying element, and is connected to a passive device for preventing laser oscillation and outputting the amplified signal, such as inline optical isolator 8. In operation, input signal light is multiplexed with pumping light in the multiplexer and amplified during transmit through the rare earth-doped fiber to output the increased signal light.
Conventionally, the multiplexer 7 has been formed of a single mode fiber and, in fabricating an amplifier, must have been connected to the components including the rare earth-doped fiber and the pumping light source. Such a structure have needed labor to connect between the optical fibers from the optical components by fusion of ends of the fibers and have had light connection loss at such connection portions between the fibers, resulting in undesirable amplification characteristics including low signal gain and noise figure properties.
In the example as shown in FIG. 7, there have been required three splicing portions 9 between multiplexer 7 and pumping light source 6, multiplexer 7 and rare earth-doped optical fiber 4, and said fiber 4 and isolator 8. These splicing portions not only have reduced light energy therethrough, but also have required spaces in which to be placed in packing an optical circuit, then increasing a packing volume for the amplifier.
Further, in the conventional amplifier structure shown in FIG. 7, the input signal light and pumping light are multiplexed in the multiplexer and thereafter entered into the rare earth-doped fiber. Since the signal light and pumping light have different mode field diameters, it has been difficult to adapt and communicate both multiplexed components of the signal light and the pumping light in optimum conditions at the splicing portion.
An object of the invention is to provide an optical fiber coupler being capable of being connected with a rare earth-doped fiber as an optical amplifying element directly without using splicing portions for fabricating an optical amplifier.
Another object of the invention is to provide an optical fiber coupler suitable for reducing connecting loss of light energy at each of fiber joint portions to constitute an optical amplifier.
Further another object of the invention is to provide an optical fiber coupler to eliminate the need for adjusting mode field diameters between signal light and pumping light transmitting through a rare earth-doped fiber.
Further another object of the invention is to provide a process for fabricating an optical fiber coupler by directly jointing optical fibers without using splicing portions.
Further another object of the invention is to provide an optical amplifier capable of being fabricated without using any splicing portions.
Further still another object of the invention is to provide an optical amplifier to reduce connecting loss of light energy at fiber joint portions to constitute an optical amplifier.
In the invention, an optical fiber coupler is provided wherein a rare earth-doped fiber is jointed with a fused-stretch fiber portion as a multiplexing element that is formed of a quasi rare earth-doped fiber with the rare earth-doped fiber by fusing and elongating both fibers.
Particularly, the optical fiber coupler of the invention comprises: a rare earth-doped fiber; a quasi rare earth-doped fiber; a fused-stretch fiber portion as a multiplexing element which connects a part of the rare earth-doped fiber with a part of the quasi rare earth-doped fiber; and a single mode fiber which is connected via a shortened side of the rare earth-doped fiber connected to the fused-stretch fiber portion.
In the optical fiber coupler, the fused-stretch fiber portion is formed by stretching the fused portion of paralleled parts of the rare earth-doped fiber and the quasi rare earth-doped fiber.
Particularly, in the invention, a quasi rare earth-doped fiber is defined as a fiber having a substantially equal propagation constant to the rare earth-doped fiber without substantially containing rare earth elements.
Particularly, the joint portion of the single mode fiber to the may be set to being in shortened distance from the fused-stretch fiber portion, then lowering light loss during passing through the shortened rare earth-doped fiber.
By the present invention, a process of fabricating an optical fiber coupler is provided, which comprises: jointing a single mode fiber with a rare earth-doped fiber by fusing abutted end faces of both fibers to form a combined fiber; and fusing parallel contact parts of a quasi rare earth-doped fiber and of the rare earth-doped fiber of the combined fiber and then elongating the fused parts in a desired diameter to form a fused-stretch fiber portion.
Also, the above optical fiber coupler may be applied to optical fiber amplifiers. In the amplifier of the invention, signal light enters the single mode fiber, pumping light enters the quasi rare earth-doped fiber to multiplexes with the signal light, and an amplified light is output from the rare earth-doped fiber.
In the optical amplifier, preferably, a pumping light source may be directly connected to the quasi rare earth-doped fiber.
In the invention, an optical amplifier is provided using two optical fiber couplers, wherein the common rare earth-doped fiber has the two optical fiber couplers, signal light enters the single mode fiber of one of the optical fiber couplers and pumping light from two pumping light sources enters both quasi rare earth-doped fibers of the optical fiber couplers, and an amplified light is extracted from the single mode fiber of the other optical fiber coupler.